1. Field of the Invention
This invention relates to a solid-state image pickup device, a driving method for a solid-state image pickup device and an image pickup apparatus.
2. Description of the Related Art
In recent years, solid-state image pickup devices such as a CCD (Charge Coupled Device) type image sensor and a CMOS (Complementary Metal Oxide Semiconductor) type image sensor are used widely as an image inputting device or image pickup device for an image pickup apparatus incorporated in various portable terminal equipments such as a portable telephone set or image pickup apparatus such as a digital still camera or a digital video camera.
FIG. 10 is a circuit diagram showing an example of a configuration of a unit pixel of a general solid-state image pickup device. Referring to FIG. 10, the unit pixel 100 shown has a circuit configuration wherein it includes four transistors in addition to a photoelectric conversion element such as, for example, a photodiode 101. The four transistors include, for example, a transfer transistor 102, a reset transistor 103, an amplification transistor 104 and a selection transistor 105. In the circuit configuration shown in FIG. 10, for example, an n-channel MOS transistor is used for the transistors 102 to 105.
The transfer transistor 102 is connected between the cathode electrode of the photodiode 101 and a floating diffusion (FD) portion 106 and has a gate electrode connected to a transfer control line 111 to which a transfer gate pulse TG is applied. The reset transistor 103 is connected at the drain electrode thereof to a power supply Vdd, at the source electrode thereof to the FD portion 106, and at the gate electrode thereof to a reset control line 112 to which a reset pulse RS is applied.
The amplification transistor 104 has a source follower circuit configuration with the gate electrode thereof connected to the FD portion 106 and with the source electrode thereof connected to a signal line 123. The signal line 123 is connected at one end thereof to a constant current source 124. The selection transistor 105 is connected at the drain electrode thereof to the power supply Vdd, at the source electrode thereof to the drain electrode of the amplification transistor 104, and at the gate electrode thereof to a selection control line 113 to which a selection pulse SEL is applied.
FIG. 11 is a sectional view showing a cross sectional structure of the unit pixel except the amplification transistor 104 and the selection transistor 105.
Referring to FIG. 11, n-type diffusion regions 132, 133 and 134 are formed on a surface layer portion of a p-type substrate 131. A gate electrode 135 is formed at a portion of the p-type substrate 131 between the n-type diffusion region 132 and the n-type diffusion region 133 with a gate oxide film (SiO2) not shown interposed therebetween. Another gate electrode 136 is formed at another portion of the p-type substrate 131 between the n-type diffusion region 133 and the n-type diffusion region 134 with the gate oxide film interposed therebetween.
In a corresponding relationship to FIG. 10, the photodiode 101 is formed by a pn junction between the p-type substrate 131 and the n-type diffusion region 132. The transfer transistor 102 is formed from the n-type diffusion region 132, the n-type diffusion region 133 and the gate electrode 135 between the n-type diffusion region 132 and the n-type diffusion region 133. The reset transistor 103 is formed from the n-type diffusion region 133, the n-type diffusion region 134 and the gate electrode 136 between the n-type diffusion region 133 and the n-type diffusion region 134.
The n-type diffusion region 133 forms the FD portion 106 and is electrically connected to the gate electrode of the amplification transistor 104. The power supply Vdd is applied to the n-type diffusion region 134 which forms a drain region of the reset transistor 103. The upper face of the p-type substrate 131 is covered with a light intercepting layer 137 except the photodiode 101.
Now, circuit action of the unit pixel 100 is described with reference to the sectional view of FIG. 11 and a waveform diagram of FIG. 12.
If light is irradiated on the photodiode 101 as seen in FIG. 11, then pairs of an electron (−) and a hole (+) are induced in response to the intensity of the light, in other words, photo-electric conversion is performed. Further, a selection pulse SEL is applied to the gate electrode of the selection transistor 105 at time T1 in FIG. 12, and a reset pulse RS is applied to the gate electrode of the reset transistor 103 at the same time. As a result, the reset transistor 103 is placed into a conducting state, and the FD portion 106 is reset to the power supply Vdd at time T2.
When the FD portion 106 is reset, the potential of the FD portion 106 upon such resetting is outputted as a reset level Vn to the signal line 123 through the amplification transistor 104. This reset level Vn corresponds to a noise component unique to the unit pixel 100. The reset pulse RS keeps an active state of the “H” level within a predetermined period from time T1 to time T3. The FD portion 106 keeps its reset state also after the reset pulse RS changes over from the active state into an inactive state of the “L” level. The period within which the FD portion 106 remains in the reset state is a reset period.
Thereafter, while the selection pulse SEL remains in the active state, a transfer gate pulse TG is applied to the gate electrode of the transfer transistor 102 at time T4. Consequently, the transfer transistor 102 is placed into a conducting state, and signal charge produced by photoelectric conversion by the photodiode 101 and accumulated till then is transferred to the FD portion 106. As a result, the potential of the FD portion 106 varies in response to the amount of the signal charge in a period from time T4 to time T5. The potential of the FD portion 106 at this time is outputted as a signal level Vs to the signal line 123 through the amplification transistor 104 (signal readout period). Then, a difference RSI1 between the signal level Vs and the reset level Vn presents a pure pixel signal level free from any noise component.
Usually, when an image of a bright object is picked up, the amount of charge accumulated in the photodiode 101 within the reset period becomes greater than that when an image of a dark object is picked up, and therefore, the difference RSI1 on the signal line 123 is greater.
(Mechanism in Occurrence of a Black Sun Phenomenon)
Incidentally, it is known that a black sun phenomenon occurs with a solid-state image pickup device. The black sun phenomenon is a phenomenon that, when very intense light such as the sunlight enters the pixel 100, the brightest portion is depressed dark.
A mechanism in occurrence of a black sun phenomenon is described with reference to FIGS. 13 and 14. FIG. 13 is a schematic view illustrating a mechanism in occurrence of a black sun phenomenon and shows a structure substantially similar to that shown in FIG. 11. FIG. 14 is a waveform diagram of the unit pixel shown in FIG. 13 when a black sun phenomenon occurs.
Within a reset period, a selection pulse SEL is applied to the gate electrode of the selection transistor 105 at time T1′ and a reset pulse RS is applied to the gate electrode of the reset transistor 103 at the same time in a similar manner as in the unit pixel shown in FIG. 11. As a result, the reset transistor 103 is placed into a conducting state, and the FD portion 106 is reset to the power supply Vdd at time T2′. The potential of the FD portion 106 upon such resetting is outputted as a reset level Vn to the signal line 123 through the amplification transistor 104.
However, if very intense light such as the sunlight is irradiated upon the photodiode 101 as seen in FIG. 13, then a large number of pairs of an electron (−) and a hole (+) when compared with those in the unit pixel of FIG. 11 are induced in a pn junction portion formed by the p-type substrate 131 and the n-type diffusion region 132. As a result, surplus electrons produced by the photoelectric conversion overflow from the photodiode 101. As a result, although the transfer gate pulse TG is in an inactive state, the surplus electrons arrive at the FD portion 106 across the transfer transistor 102. Therefore, the potential of the FD portion 106 drops, and as a result, the potential of the signal line 123 drops in a period from time T2′ to time T4′.
Similarly, if a transfer gate pulse TG is applied to the gate electrode of the transfer transistor 102 at time T4′ while the selection pulse SEL remains in an active state within a signal readout period, then the transfer transistor 102 is placed into a conducting state. Consequently, signal charge produced by photoelectric conversion by the photodiode 101 and accumulated is transferred to the FD portion 106. As a result, the potential of the FD portion 106 varies in response to the amount of the signal charge in a period from time T4′ to time T5′. The potential of the FD portion 106 at this time is outputted as a signal level Vs to the signal line 123 through the amplification transistor 104.
At this time, as a result of the overflowing of surplus electrons within the reset period as described above, the potential of the signal line 123 exhibits a drop when compared with that upon application of the reset pulse RS as seen apparently from FIG. 14. As a result, the potential difference RS12 within the signal readout period drops although the intense light is irradiated.
FIG. 15 is a diagrammatic view illustrating a mechanism in occurrence of a black sun phenomenon. Referring to FIG. 15, the axis of abscissa indicates the incident light amount to the pixel 100 and the axis of ordinate indicates the pixel signal amount obtained from the pixel 100.
In a usual state, the difference Vsig-Vres between the signal level Vsig within a signal readout period and the reset level Vres within a reset period is outputted as a pixel signal level. If the incident light amount exceeds a fixed light amount B, then the signal level Vsig is saturated and a fixed pixel signal level is outputted.
Then, the light leak noise Vn becomes a gradually increasing signal at a predetermined light amount C which is greater than the light amount B. Then, if a differencing process is performed within a region within which the light amount C is exceeded, then the difference Vsig-Vres exhibits a reduced value although the intense light is irradiated. Therefore, although the image pickup object is very bright, a black sun phenomenon wherein it looks dark occurs.
Here, the light leak noise Vn is described. When intense excessively high light comes in, charge accumulated in the photodiode 101 which has a function of converting light into electrons may overflow, and the overflowing charge makes noise. Or, when light leaks into the FD portion 106 which has a function of converting charge read out from the photodiode 101 into an electric signal, the leaking-in component makes noise.
In order to prevent a black sun phenomenon, in the past, it is decided whether or not the incident light is very intense by detecting whether the signal level Vsig is within a saturation region wherein the light amount B is exceeded and whether the reset level Vres is within a region within which it varies, that is, within a region wherein the light amount C is exceeded. Then, the differencing process Vsig-Vres is corrected based on a result of the decision. This method is disclosed, for example, in Japanese Patent Laid-Open No. 2004-248304.
In particular, such a countermeasure as shown in FIG. 16 is taken. Referring to FIG. 16, a switch 206 is interposed in a signal line for a reset level Vres between an amplifier 204 and a differential amplifier 205. The amplifier 204 amplifies a reset level Vres and a signal level Vsig outputted from a pixel array section 201 through an N memory 202 and an S memory 203, respectively. The differential amplifier 205 differentially amplifies the signal level Vsig and the reset level Vres from the N memory 202 and the S memory 203, respectively. Then, if a light level detection circuit 207 detects that the signal level Vsig is equal to or higher than a predetermined level Va or the reset level Vres is equal to a predetermined level Vb, then the switch 206 is switched off to stop the differential amplification process of the differential amplifier 205 to prevent a black sun phenomenon.
Or, such another countermeasure as shown in FIG. 17 is taken. Referring to FIG. 17, also when the light level detection circuit 207 detects that the signal level Vsig is equal to or higher than the predetermined level Va or the reset level Vres is equal to the predetermined level Vb, the differential amplifier 205 performs a differential amplification action. Then, a resulting difference by the differential amplifier 205 is converted into a digital signal by an A/D conversion circuit 208 and stored into a memory 209. On the other hand, upon detection of the state described hereinabove by the light level detection circuit 207, when the difference signal is read out from the memory 209, the difference signal is converted into a signal of a predetermined level, that is, the level Va, by a conversion circuit 210 or digital data of the A/D conversion circuit 208 are converted into saturation data in response to a saturation detection signal of the light level detection circuit 207 to prevent a black sun phenomenon.